1. Field of the Invention
The present invention relates to a lithographic plate material with an image-receptive layer capable of providing water retention characteristics suitable for a lithographic fountain solution without desensitizing. In particular, it relates to a lithographic plate material capable of forming lipophilic images using an ink-jet printer with hot-melt type solid ink.
2. Related Art
Lithographic printing using a lithographic printing plate is widely used for printing a small number of printed items, e.g., less than ten thousand. Conventionally, the lithographic printing plates are made by forming an image-receptive layer on a substrate such as a waterproof paper and printing a lipophilic image on the image-receptive layer using a typewriter or making a copy of an original block copy using a dry elecrophotography copier to form lipophilic images of toner on the image-receptive layer.
With the recent development of computers and peripheral devices, plate making processes using various kinds of digital printers have been proposed. For example, Japanese patent application laid-open Nos. 6-138719 (94), 6-250424 (94) and 7-1847 (95) disclose methods for making a lithographic printing plate by forming toner images on an image-receptive layer using a dry-type electrophotography laser printer.
The inventions disclosed in the aforementioned patent applications are directed to reduction of scumming in non-image portions of a print, which is likely to occur when dry-type electrophotography laser printers are used, and to obtain a lithographic plate having excellent plate wear.
Japanese patent application laid-open No.9-58144 (97) discloses a method of making a lithographic plate which does not use the dry-type electrophotography laser printer having such a drawback but, rather, forms lipophilic images on an image-receptive layer using an ink-jet printer with a hot-melt type solid ink.
However, the image-receptive layer of the lithographic plate disclosed in the above-mentioned patent application contains zinc oxide and a polymer binder as main components and, therefore, it is necessary to sufficiently desensitize the zinc oxide at the surface of the image-receptive layer in order to impart sufficient water retention to enable application of lithography to the image-receptive layer by means of an etching process using a cyan-system etching solution containing phosphoric acid/potassium ferricyanide as a main component or a non-cyan system etching solution containing phytic acid as a main component.
Despite efforts to improve image-receptive layers or printing methods using digital printers to reduce generation of scumming in non-image portions of a print, insufficient desensitization is achieved in the plate making process due to degradation of the aforementioned etching solution and lowering of the liquid temperature and, as a result, scumming occurs.
In addition, a printing process using the lithographic plate disclosed in the above-mentioned application requires a special fountain solution containing components similar to those of the etching solution. As a result, when the plate is used together with another lithographic plate such as an aluminum graining presensitized (PS) plate and/or lithographic plate made by a silver salt diffusion transfer process, the fountain solution in the lithographic press has to be changed. This makes printing more problematic.
The present invention aims at solving the abovementioned problems. Specifically, an object of the present invention is to provide a lithographic plate material which can be desensitized with distilled water or a fountain solution used for other lithographic plates, without using an etching solution, and to reduce scumming in non-image portions of the printed product. Another object of the present invention is to provide a method of making a lithographic plate using the novel lithographic plate material.
The lithographic plate material of the present invention comprises a support and an image-receptive layer formed on the support, wherein the surface of the image-receptive layer has an arithmetic mean roughness defined by JIS (Japanese Industrial Standard)-B0601 of not less than 0.40 xcexcm and less than 1.20 xcexcm, a contact angle with distilled water at room temperature of less than 50 degrees, and the image-receptive layer has ink-receptiveness for hot-melt and lipophilic ink.
The lithographic plate material of the present invention has an image-receptive layer, a hydrophilic polymer binder and inorganic microparticles.
Preferably, the lithographic plate material of the present invention has an image-receptive layer comprising polyvinyl alcohol cross-linked by hydroxylate of tetra-alkoxy silane, titanium oxide microparticles, and silica having an average primary particle size of from 1 nm to 100 nm and/or alumina having an average primary particle size of from 1 nm to 100 nm.
The lithographic plate material of the present invention may further include an undercoat layer between the support and the image-receptive layer. In this lithographic plate material, the undercoat layer may contain inorganic microparticles or synthetic resin microparticles.
The method of making a lithographic plate of the present invention comprises forming lipophilic images on the image-receptive layer of the lithographic plate material using hot-melt and lipophilic ink.
The method of making a lithographic plate of the present invention may further include imparting water retention characteristics for a lithographic fountain solution to the surface of the image-receptive layer without a desensitizing process.
The method of making a lithographic plate of the present invention may form lipophilic images on the image-receptive layer using an ink-jet printer with hot-melt solid ink.
Preferred embodiments of the present invention will be explained in detail hereinafter. In the following explanation, xe2x80x9cpartxe2x80x9d and xe2x80x9c%xe2x80x9d are used on a weight basis unless otherwise indicated.
The lithographic plate material of the present invention comprises a support and an image-receptive layer formed on the support.
As the support, a plastic film composed of a resin such as polyethylene, polypropylene, polyvinylchloride, polystyrene, polyethylene-terephthalate, waterproof paper having such a plastic film laminated thereon or waterproof paper coated with such a resin can be used.
A polyethylene-terephthalate film is particularly preferred in view of its mechanical strength, dimensional stability, resistance to chemicals, and waterproof property. The support may include a light-shielding pigment such as carbon black or titanium oxide in order to make it light-shielding. The thickness of the support may be not less than 50 xcexcm and less than 300 xcexcm.
In order to improve adhesiveness to the image-receptive layer, the support is preferably exposed to far ultraviolet rays, or subjected to a plasma process, a corona discharge process or, preferably, an undercoating process.
Materials of the undercoat depend on the type of support employed. When polyethylene-terephthalate film is employed, it may be formed by applying a coating solution containing isocyanate prepolymer dissolved in a resin selected from acetal resins such as polyvinylbutyral, polyester resins having a terminal hydroxyl group and acrylic copolymers having side chain with a terminal hydroxyl group, to the support so that the undercoat has a dry thickness of not less than 0.5 xcexcm and less than 10 xcexcm.
To improve adhesiveness between the support and the image-receptive layer and to adjust the surface roughness of the image-receptive layer to be laminated thereon, the undercoat may contain inorganic microparticles such as calcium carbonate, barium sulfate, silica, zinc oxide, titanium oxide, clay, alumina or synthetic resin microparticles such as acrylic resin, epoxy resin, nylon resin, polyethylene resin, fluorine resin, or benzoguanamine, in an amount of not less than 5 parts and less than 200 parts based on 100 parts of the binder resin of the undercoat.
Within the above-mentioned range, the surface of the undercoat can be roughened without lowering the film strength of the undercoat. This roughened surface affects the surface of the image-receptive layer and gives a preferred surface roughness to the surface of the image-receptive layer.
The coating solution for the undercoat can be prepared using, as occasion demands, known means for preparing a dispersion such as a ball mill, sand grinder, attritor, roll mill, or high-speed impeller dispersion mixer. The undercoat can be formed by applying the aforementioned coating solution on the support by means of a known coating method such as roll coating, bar coating, or blade coating with heat drying at a predetermined temperature.
The image-receptive layer prepared on a support is required to have an arithmetic mean surface roughness defined by JIS-B0601 of not less than 0.40 xcexcm and less than 1.20 xcexcm in order to have an ink-receptiveness for hot-melt solid ink. In addition to the surface roughness of the aforementioned range, it is also required to have a contact angle of less than 50 degrees to distilled water at room temperature in order to have water retention characteristics suitable for a lithographic fountain solution. Such an image-receptive layer consists of at least a hydrophilic polymer binder and inorganic microparticles.
Examples of the hydrophilic polymer binder include polyvinyl alcohol, carboxymethyl cellulose, hydroxyethyl cellulose, polyvinylpyrrolidone, and methylvinyl ether/maleic anhydride copolymer. Completely saponified polyvinyl alcohol having a polymerization degree of less than 1000 and a saponification degree of 97 mol% or more is preferred since inorganic microparticles can be dispersed therein very well during preparation of the coating solution of the image-receptive layer and because it can be cross-linked by a tetra alkoxy silane hydroxylate, which will be explained hereinafter.
The tetra alkoxy silane used in the present invention can be obtained by allowing silane tetra chloride, which is a reaction product of metallic silicon (a reduction product of silica) and chlorine, to react with alcohol. The tetra alkoxy silane is further hydrolyzed in a mixture of water and alcohol, such as ethanol, isopropanol, or ethyl cellosolve in the presence of a catalyst acidified with HCl to provide the hydroxylate.
The tetra alkoxy silane hydroxylate functions as a crosslinker of the polyvinyl alcohol contained in the image-receptive layer through a silanol group included in its molecule and thereby makes the image-receptive layer waterproof and imparts thereto suitable water retention characteristics.
The amount of the tetra alkoxy silane hydroxylate added to the polyvinyl alcohol is preferably 20 parts or more and less than 200 parts based on 100 parts of polyvinyl alcohol in terms of the amount of tetra alkoxy silane before hydrolysis. With an amount of 20 parts or more, an excellent waterproof property can be obtained. With an amount of less than 200 parts, degradation of water retention characteristics suitable for a lithographic fountain solution can be prevented and generation of scumming in non-image portions of printed products can be reduced.
In order to further improve water retention and waterproof characteristics and mechanical strength, water soluble resins such as carboxymethyl cellulose, hydroxyethyl cellulose, polyvinylpyrrolidone, and methylvinylether/maleicanhydride copolymer, and emulsions of homopolymers or copolymers of vinyl chloride, vinyl acetate, acrylic ester, ethylene, styrene and the like can be added to the polyvinyl alcohol in an amount of less than 30 parts based on 100 parts of the polyvinyl alcohol.
With an amount less than 30 parts, water retention and waterproof characteristics and mechanical strength can be improved without impairing the characteristics of the polyvinyl alcohol. The aforementioned emulsion can be a complex with colloidal silica or the like so as to further improve the water retention, water proof characteristics and mechanical strength of the image-receptive layer.
Examples of inorganic microparticles used in the present invention include calcium carbonate, barium sulfate, silica, titanium oxide, clay, alumina and the like. In order to impart ink-receptiveness for hot-melt and lipophilic ink, water retention characteristics and mechanical strength to the image-receptive layer, it is preferred to use a combination of titanium-oxide microparticles and silica having a primary particle size of not less than 1 nm and less than 100 nm and/or alumina having a primary particle size of not less than 1 nm and less than 100 nm, which have good adhesiveness to hot-melt and lipophilic ink, show good water retention between molecules and are excellent in mechanical strength.
The titanium-oxide microparticles are prepared by the sulfuric-acid method or chlorine method. The crystalline form may be a rutile form or an anatase form. The average particle size may be not less than 0.05 xcexcm and less than 1.0 xcexcm. The surface of the particles may be processed with alumina or silica to become hydrophilic.
The amount of the titanium oxide is 50 parts or more, and preferably 100-1000 parts, preferably less than 800 parts, based on 100 parts of the polymer binder included in the image-receptive layer.
Within these ranges of average particle size and amount, the image-receptive layer can have an arithmetic mean surface roughness defined by JIS-B0601 of 0.40 xcexcm or more and less than 1.20 xcexcm to provide ink-receptiveness for hot-melt and lipophilic ink, water retention characteristics suitable for a fountain solution, waterproof property and mechanical strength.
A silica having a primary particle size of not less than 1 nm and less than 100 nm is anhydrous silica prepared by a dry process and having a hydrophilic surface with many silanol groups. The lower limit of the primary particle size is 1 nm, preferably 5 nm, and more preferably 10 nm. The upper limit is 100 nm, preferably 40 nm, and more preferably 20 nm. The BET specific surface area is not less than 40 m2/g, and preferably 100 m2/g-400 m2/g, preferably less than 300 m2/g.
Within these ranges, water retention suitable for a fountain solution can be improved without degrading adhesiveness of the image-receptive layer to hot-melt and lipophilic ink. The silica may be a mixed oxide of silica and alumina.
Alumina having a primary particle size of not less than 1 nm and less than 100 nm is also prepared by a dry process. The lower limit of the primary particle size is 1 nm, and preferably 10 nm. The upper limit is 100 nm, and preferably 40 nm. The BET specific surface area is preferably not less than 40 m2/g and less than 200 m2/g.
Within the above ranges, adhesiveness of the image-receptive layer to hot-melt and lipophilic ink can be improved without degrading the water retention property of the image-receptive layer. Silica and alumina may be used alone or as a mixture.
The amount of silica and/or alumina is not less than 2 parts, and preferably not less than 5 parts and less than 200 parts, preferably less than 100 parts based on 100 parts of the polymer binder included in the image-receptive layer. Within this range of amounts, silica and/or alumina can be well dispersed in the coating solution for forming the image-receptive layer together with titanium oxide microparticles. Therefore, the coating solution can be applied easily. In addition, the image-receptive layer can have good water retention property, ink-receptiveness for hot-melt and lipophilic ink, waterproofness and mechanical strength.
As occasion demands, the image-receptive layer may contain inorganic microparticles such as calcium carbonate, barium sulfate, clay, silica and alumina having different particle sizes, and the like or synthetic resin microparticles such as acrylic resins, epoxy resins, nylon, polyethylene resins, fluororesins, and benzoguanamine resins, in an amount of not more than 100 parts based on 100 parts of the polymer binder included in the image-receptive layer. Silica having an average particle size of not less than 1 xcexcm and less than 10 xcexcm is particularly preferred since it gives fine irregularity to the surface of the image-receptive layer and improves water retention suitable for a fountain solution without degrading waterproofness.
The total amount of titanium oxide microparticles, silica and/or alumina having a primary particle size of not less than 1 nm and less than 100 nm, optional inorganic microparticles and synthetic resin microparticles is not less than 80 parts and preferably 100 to 1000 parts, preferably less than 800 parts, based on 100 parts of the polymer binder included in the image-receptive layer.
If the above amount is less than 80 parts, the image-receptive layer will not have sufficient surface irregularity and, therefore lipophilic images formed thereon by hot-melt solid ink will not adhere properly and tend to be removed during the printing process. This results in poor print durability. In addition, the image-receptive layer has insufficient water retention property for a fountain solution, which leads to generation of scumming in non-imaged portions of the printed product.
On the other hand, if the amount is 1000 or more parts, the surface of the image-receptive layer becomes too rough, and ink-receptiveness for lipophilic images by hot-melt solid ink becomes uneven. This results in low resolution and poor image reproducibility in the printed product. In addition, the film strength of the image-receptive layer is degraded and part of the layer is removed from the support during the printing process. This leads to generation of scumming and poor printing durability.
To form the image-receptive layer of the lithographic plate material of the present invention, an aqueous solution containing not less than 5% and less than 20% of the aforementioned solution of polyvinyl alcohol in distilled water is prepared, and titanium oxide microparticles, silica and/or alumina having a primary particle size of not less than 1 nm and less than 100 nm, optional inorganic microparticles and synthetic resin microparticles, water soluble resins, and homopolymer or copolymer resin emulsion are mixed therein. Then tetra alkoxy silane is added thereto to obtain a coating solution. The coating solution is applied to a support by means of a conventional coating method such as roll coating, bar coating, or blade coating to form a coating layer on the support. The coating layer is dried in an atmosphere of not less than 50xc2x0 C. and less than 200xc2x0 C. for 30 seconds to 10 minutes.
Optionally, the aforementioned coating solution for the image-receptive layer may be prepared using a known means for preparing a dispersion such as a ball mill, sand grinder, attritor, roll mill, or high-speed impeller dispersion mixer.
The lithographic plate material of the present invention may be provided with a low electric-resistance layer on the side opposite the aforementioned image-receptive layer. The low electric-resistance layer helps static chucking in an ink-jet printer using hot-melt solid ink, which will be explained later, and consequently the lithographic plate material of the present invention can be attached to a vertical mounting portion without a clamp or adhesive tape. In addition, since the lithographic plate material of the present invention adheres to the mounting portion, flatness of the lithographic plate can be kept without floating, and rubbing of the image receiving surface with a recording head can be prevented.
The low electric-resistance layer consists of an ion conductive acrylic resin obtained by copolymerization of a cationic monomer having quaternary ammonium-salt groups such as hydroxy propyltrimethyl ammonium chloride methacrylate and oxyethyl trimethyl ammonium chloride methacrylate, and a lipophilic monomer such as methyl methacrylate and butyl methacrylate.
The surface resistivity of the aforementioned low electric-resistance layer is preferably less than 1010xcexa9. If surface resistivity is 1010xcexa9 or more, the static mounting mechanism does not operate efficiently.
A method of making a lithographic plate of the present invention comprises providing the aforementioned lithographic plate material and forming lipophilic images on the image-receptive layer of the material using hot-melt and lipophilic ink.
To apply the hot-melt and lipophilic ink on the image-receptive layer, ink-jet printers using hot-melt type solid ink can be employed.
The ink-jet printer using hot-melt type solid ink is disclosed in xe2x80x9cElectrophotography No.112 pp71-75 (Society of Electrophotography)xe2x80x9d and xe2x80x9cJapan Hardcopy ""99, Literatures pp.347-350 (The Imaging Society of Japan).xe2x80x9d The ink-jet printer forms lipophilic images by heating and liquefying solid ink, which is solid at room temperature, and ejecting recording dots toward the image-receptive layer.
The hot-melt type solid ink is composed mainly of a hot-melt compound, i.e. wax, which is solid at room temperature and liquefied by heating. In order to obtain suitable printing characteristics for ink-jet printing, it may further contain resins selected from polyamide resins, polyester resins, polyvinyl acetate resin and the like, and colorant. In addition, acrylate resins, urethane resins and the like may be added to the hot-melt type solid ink so that the ink adheres firmly to the image-receptive layer and mechanical strength, such as surface abrasion strength of the lipophilic images, chemical strength such as resistance to printing ink or a fountain solution, and affinity for the printing ink can be maintained.
According to the method of making a lithographic plate of the present invention, the surface of the image-receptive layer can be imparted with water retention using a lithographic fountain solution without desensitizing the layer. Consequently, a lithographic plate made from the material can be directly used in a lithographic press. As occasion demands, however, distilled water or a lithographic fountain solution may be applied to the image-receptive layer to prevent the plate surface from becoming stained.